CCELENTERATA : ACTINOZOA. 103 



form an axial column, called the ' columella.' Many of the 

 septa, however, do not reach the centre, but stop short at 

 some distance from the columella, often being broken up into 

 upright pillars, called 'pali.' The parts thus described as 

 essentially composing a corallite in a typical sclerodermic co- 

 rallum are related in the most obvious manner to the soft 

 structures of the animal by which they are secreted. Thus, 

 the ' theca ' clearly corresponds to the ' column- wall,' or the 

 general wall of the body ; the ' columella,' when present, cor- 

 responds to ' that part of the enderon which forms the floor of 

 the somatic cavity below the digestive sac,' whilst the 'septa' 

 correspond to the ' mesenteries,' and, like them, are called 

 'primary' and 'secondary,' according as they reach the 

 columella or fall short of it. When there are several corallites 

 the bond of union between them, the 'ccenenchyma,' is secreted 

 by the ' ccenosarc,' to which it corresponds. In many Actinozoa, 

 however, the sclerodermic corallum is not present in the 

 typical form above described, but simply in the form of cal- 

 careous spicules or nodules scattered through the tissues of 

 the animal. There are, also, members of the class in which 

 both a sclerodermic and a sclerobasic corallum are present, 

 the latter constituting the main skeleton, whilst the former is 

 represented by scattered spicules. The coral tissue itself is 

 known as ' sclerenchyma,' and it varies considerably in texture, 

 being sometimes extremely compact, and at other times very 

 loosely put together. 



From what has been said it will be seen that a sclerobasic 

 corallum can easily be distinguished from a sclerodermic by 

 inspection ; the former being usually more or less smooth, and 

 being invariably devoid of the cups or receptacles for the se- 

 parate polypes, which are always present in the latter. The 

 more important variations of detail which occur in both 

 classes of corals will be noticed under the different families in 

 which they occur. 



Returning now to the Zoantharia Sclerolasica, we find the 

 sub-order to contain the two families of the Antipathidce and 

 the Hyalonemadce (or Hyalochcetidce) . Of these the Antipathidce 

 are chiefly noticeable because of their likeness to some of the 

 Gorgonidcv, from which, however, they are readily distinguished 

 by the fact that the number of their tentacles is a multiple of 

 six, whereas in the latter it is a multiple of four. Antipatlies 

 itself possesses a horny sclerobasic corallum, which may be 

 simple or branched, and is covered with numerous small 

 polypes, united together by a ccenosarc, and possessing six 

 tentacles each. 



The second family, that of the Hyalonemadce, contains the 



